21cc is safe and allows room for timing adjustments.. 20cc is a gamble without knowing the setup. By setup.. i mean base gaskets, jetting, quality of fuel and Dome design. Not all 20cc domes are the same.. some are more prone to deto then others. I ride around 600ft elevation. I typically use 21cc domes and a plate on setups that dont want a 50/50 mix of fuel.